Multi-cell batteries are typically constructed in a broad range of electrochemical systems and are often packaged in cylindrical or prismatic housings. Individual cells are connected in series by conductive links to make the multi-cell batteries. Such construction approaches provide for good sealing of the individual cell compartments and for reliable operation. However, such constructions allocate a large fraction of the multi-cell battery's weight and volume to the packaging and, thus, do not make full use of the energy storage capability of the active components of the cell. For improving battery energy storage capacity on a weight and volume basis, packaging approaches are sought that reduce packaging weight and volume and that provide stable battery performance and low internal resistance.
These objectives have led to the pursuit of a bipolar construction in which an electrically conductive bipolar layer serves as the electrical interconnection between adjacent cells, as well as a partition between the cells. In this type of construction, the current flows perpendicular from cell to cell over the entire cell area thus increasing high rate capability. However, in order for the bipolar construction to be successfully utilized, the bipolar layer should be sufficiently conductive to transmit current from cell to cell, chemically stable in the cell's environment, capable of making and maintaining good electrical contact to the electrodes, and capable of being electrically insulated and sealable around the boundaries of the cell so as to contain electrolyte in the cell. These features are more difficult to achieve in rechargeable batteries due to the charging potential that can accelerate corrosion of the bipolar layer and in alkaline batteries due to the creep nature of the electrolyte. Achieving the proper combination of these characteristics has proven to be very difficult.
For maintenance-free operation, it is desirable to operate rechargeable batteries in a sealed configuration. However, sealed bipolar designs typically utilize flat electrodes and stacked-cell constructions that may be structurally poor for containment of the gases present or generated during cell operation. In a sealed cell construction, gases are generated during charging that need to be chemically recombined within the cell for stable operation. To minimize weight of the structures used to provide the gas pressure containment, the battery should operate at relatively low pressure. The pressure containment requirement creates additional challenges on designing a stable bipolar configuration.
Also, the need for removal of heat generated during normal operation of batteries may be a limiting design factor in bipolar construction due to the compact nature of the construction. Thus, an optimum bipolar design should provide for removal of heat generated during operation.
In U.S. Pat. No. 5,393,617, electrode structures that are adaptable for primary and electrically rechargeable electrochemical wafer cells are disclosed. According to an embodiment set forth in that patent, a flat wafer cell includes conductive, carbon-filled polymeric outer layers that serve as electrode contacts and as a means of containment of the cell. Multi-cell, high voltage batteries may be constructed by stacking individual cells. Specially formulated electrodes and processing techniques that are compatible with the wafer cell construction are particularly disclosed for a nickel-metal hydride battery system. The cell design and electrode formulation disclosed in the '617 patent provide for individual operation of a vented or sealed cell and/or for operation of these cells in a stacked array in an outer battery housing.
The foregoing construction approach of the '617 patent is advantageous and has proven to be flexible for designing batteries having different capacity, voltage and chemistry. However, scientists and engineers working under the direction of Applicant's assignee are continually seeking to develop further improved wafer cell and battery constructions, and methods of fabrication thereof.